With that ISO and that long an exposure, what you are likely looking at is a hotspot near the sensor of your camera. Doesn't have to be very hot to have something show up with that ISO and shutter speed.

See if it vanishes this winter, or if it shows less on the first shot than on the third. Do the experiment to see if the effect gets worse as the camera warms up and let us know the result.

The ambient temperature doesn't help, but I suspect the problem is due to heat from the electronics in the camera. A double whammy at long exposures: the electronics are running longer so more heat builds up and there is longer for the sensor to react to the temperature. Higher ambient temperature means less heat dissipation so it is worse at higher air tempertures.

I think the solution is to open your aperature a couple of stops so you can shoot at 6 seconds instead of 25 seconds.